Fluorescent chromophores have become essential to modern chemical investigations. Chromophores with high quantum yields of emission, such as fluorescein, coumarin and arylmethine dyes, have been used in applications ranging from biological imaging and sensing to light harvesting.
Pyrrole based chromophores have long played central roles in chemistry and related fields. Fluorescent chromophores have become essential to modern chemical investigations. Chromophores with high quantum yields of emission, such as fluorescein, coumarin and arylmethine dyes, have been used in applications ranging from biological imaging and sensing to light harvesting.
The archetypal polypyrrole is porphyrin, a tetrapyrrolic aromatic macrocycle that is common in biology (as an enzyme and protein cofactor) and has a rich synthetic chemistry. Porphyrin and its metal complexes have many applications as dyes, sensors, catalysts and components of advanced materials, such as photovoltaics. Pyrrole has also been used to generate alternate macrocycles (such as corrole, porphycene, or N-confused porphyrin), conjugated polymers, and chelates.
Dipyrromethenes are non-macrocyclic, pyrrole-based chelates. These compounds can be readily generated via the condensation of two equivalents of pyrrole and one equivalent of an aldehyde (although other synthetic methods have been employed), and the resultant conjugated chelates can be stabilized by coordination to a metal or main group ion.
Some of the more successful fluorophores in the literature belong to the boron-dipyrromethene (BODIPY®) family of compounds. Also known as 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene, BODIPY is a small molecule that absorbs visible light near 500 nanometers (nm). These dyes, which are comprised of a dipyrromethene bound to a central BF2 unit, have several optimal characteristics, including a large molar absorptivity, a high quantum yield of emission, and a reasonably sized Stokes shift. Quantum yield is close to unity in both organic solvents and water, allowing BODIPY to have a wide range of applications.
The BODIPY core can be functionalized at different peripheral positions to tune its fluorescence and expand its uses. For example, the BODIPY core can be attached to various biomolecules to enhance imaging in cells and in clinical diagnosis of disease. More recently, these compounds have been investigated as potential photosensitizers. In addition to biotechnology applications, BODIPY molecules are useful as dyes in material chemistry and optics, organic light-emitting diodes (OLED), and photovoltaic materials. The success of the BODIPY dyes and related compounds has spurred investigations into similar systems, such as the nitrogen substituted aza-BODIPY variants.
The meso carbon of the BODIPY core can be replaced by a nitrogen atom to form an aza-BODIPY. The latter does share some properties of the normal BODIPY. Both BODIPY and aza-BODIPY are highly fluorescent and have a high extinction coefficient. Both also absorb strongly in the UV region but aza-BODIPY absorbance is red-shifted (>500 nm). Both BODIPY and aza-BODPY require multiple steps for their synthesis, and the precursor to BODIPY, dipyrromethene, is an unstable molecule.
In spite of the extensive chemistry of isoindoline precursors to generate phthalocyanine macrocycles, the use of this chemistry to make a phthalocyanine analog of BODIPY is largely unexplored. Such a variant would require two changes to the BODIPY skeleton: the substitution of a nitrogen atom for the bridging carbon atom position, and the conversion of the pyrrole units to isoindoline rings. To date, there have been few examples of “half-phthalocyanine” like chelates. The synthesis methods that have been reported in the literature are non-trivial, and have limited versatility.